A previously known pressure control apparatus changes a cross-sectional area of a fluid passage between a valve portion and a valve seat through relative movement of the valve portion relative to the valve seat. An armature and a rod are driven by a magnetic pulling force (also referred to as a magnetic attracting force) of a pulling force generating device, which magnetically pulls the armature along with the rod toward a body (or a yoke) of the pressure control apparatus in a valve-closing direction upon energization of a coil of the pulling force generating device. Thus, the valve portion is urged in the valve-closing direction through the armature and the rod. Furthermore, the valve portion is urged in the valve-opening direction by a fluid force and a resilient urging force of a spring (see, for example, US 2011/0127357A1).
However, when the fluid force is increased to increase the amount of lift of the valve portion from the valve seat, an air gap between the body (or the yoke) of the pressure control apparatus and the armature is increased to result in a decrease in the magnetic pulling force. Thereby, even when the fluid pressure is controlled and is thereby reduced, the valve portion does not easily move in the valve-closing direction. As a result, the fluid pressure is substantially reduced to result in an increase in the amount of change in the fluid pressure, and thereby the pressure controllability of the pressure control apparatus is deteriorated.
Furthermore, the valve portion is urged in the valve-opening direction by the spring. Therefore, in order to balance the forces, the corresponding magnetic pulling force, which corresponds to the amount of the spring force, is required besides the fluid force. Thus, a size of the coil of the pulling force generating device is disadvantageously increased, and thereby the size of the pressure control apparatus is disadvantageously increased.
The present disclosure addresses the above disadvantages. According to the present disclosure, there is provided a pressure control apparatus, which includes a body, a valve portion, a pulling force generating device, a first resilient member and a second resilient member. The body forms a fluid passage, which conducts fluid, and a valve seat, which is located in the fluid passage. The valve portion is displaceable relative to the valve seat in both of a valve-opening direction, which is away from the valve seat, and a valve-closing direction, which is toward the valve seat, to change a cross-sectional area of the fluid passage at a location between the valve seat and the valve portion. The valve portion is urged in the valve-opening direction by the fluid located on an upstream side of the valve portion in the fluid passage. The pulling force generating device generates a pulling force to urge the valve portion in the valve-closing direction. The pulling force of the pulling force generating device is adjustable to control a pressure of the fluid in the fluid passage. The first resilient member generates a resilient force to urge the valve portion in the valve-closing direction. The second resilient member generates a resilient force against the resilient force of the first resilient member. The resilient force of the first resilient member and the resilient force of the second resilient member are set to be balanced with each other when the valve portion is placed in a lift position that is between a full-closing position of the valve portion, in which the valve portion is seated against the valve seat to fully close the fluid passage, and a full-opening position of the valve portion, in which the valve portion is fully lifted away from the valve seat to fully open the fluid passage.